there are isolated packages. Like TO-220F.
But the difference in thermal resistance is huge:
0.5K/W non isolated vs 3.4K/W isolated package. (From Fairchild FDP(F)39N20 datasheet. Randomly chosen)
You can buy MOSFETs in a package (e.g. TO-220F) with the tab insulated from the die.
Otherwise you can use a thin mounting insulator to isolate the tab from the heat-sink, of course.
I guess it depends on the spcaer. There are many different types. Silicone, filled or not, ceramics, mica...i actually wonder if the TO220-F package has lower RthJC than the TO220 when the TO220 has a thermal spacer between itself and the heatsink?
It is. You determine the effective metal tab area (the part that is directly connected to the chip), divide by insulator thickness and get the "m" value...there must be some way of using the W/mK figure to get the RthJH value when its on the back of a TO247 FET ?
Can you refer to a TO247-F MOSFET on the market? Good thermal interface materials have W/mk numbers of 6 up to 10, by the way. Only Al2O3 ceramic is much better.My "thermal sense" kind of tells me that a 0.152mm thermal rubber pad with W/mK of some 0.9 would be better than using a T0247-F type FET?...ie lower therm res to heatsink?
Thanks, thats 1.5e-4/1.52e-4 = 0.987degC/W.it is. You determine the effective metal tab area (the part that is directly connected to the chip), divide by insulator thickness and get the "m" value.
....thanks, i cant find one...just looked.....no TO247 F type FETs exist...just looked on digikey over all 650-750V TO247's....that seems to answer the qu?.......F type must be worse than using spacersCan you refer to a TO247-F MOSFET on the market?
Hello. Let me share how the TO247/TO220SMPS can be connected. It is a step-by-step guideline that might be very helpful to you.Hi,
Why is it that all TO247/TO220 SMPS switching FETs have the metal tab connected internally to the drain? Surely the back tab should be connected to the Source, or not connected to anything electrically….this is because the heatsink would never be willingly connected to the drain, as it’s a switching node…and you woudlnt want your heatsink propagating the switching node?
So why cant you find FETs with tab connected to source?...or are there some?
Example of Drain connected to tab...
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